Ms. Diaz, a Worcester State University student, was fatally shot in the neck Aug. 24, 2010, while behind the wheel of her parked car on Fairfax Road during what police determined was an attempt to rob her of money and marijuana.

Mr. Smith, 20, formerly of 23 Jefferson St., shook his head in apparent disbelief after the verdicts were announced this afternoon. Martha Vassar, Ms. Diaz's mother, wept.

At the request of Assistant District Attorney Michael D. McHugh, Judge John S. McCann postponed sentencing until tomorrow and ordered that Mr. Smith remain in custody overnight without bail.

Mr. Smith is facing a mandatory sentence of life imprisonment without parole on the first-degree murder conviction.

He was one of five people charged in connection with the killing. His brother, Marcus Young, also formerly of 23 Jefferson St., is awaiting trial on murder and accessory charges in the case.

The other three defendants, William Madison, Kassie Ago and Kenny Roman, testified against Mr. Smith under cooperation agreements with the prosecution. None was charged with murder and all three said they had been promised reduced sentencing recommendations in exchange for their cooperation.

The five were arrested after police determined that the last call Ms. Diaz received on her cellphone before being shot was from a cellphone registered to Mr. Madison.

When questioned by detectives on Oct. 6, 2010, Mr. Smith admitted he was part of a plan to rob Ms. Diaz of money and marijuana and placed himself in her car moments before she was shot. He denied shooting her, however.

Ms. Ago and Mr. Madison testified that Mr. Smith left their apartment at 6 Vernon Street Place on the afternoon of Aug. 24, 2010, and went to meet Ms. Diaz on Fairfax Road.

They said he was armed with a .380-caliber semiautomatic handgun that belonged to Mr. Madison.

That weapon was never recovered by police.

Ms. Ago said Mr. Smith returned to the Vernon Street Place address a short time later saying, “I shot her. I shot her.”

According to Ms. Ago, Mr. Roman suggested Ms. Diaz would be a good target for a robbery, but warned that she might be armed, which prompted Mr. Young to ask Mr. Madison if Mr. Smith could bring Mr. Madison's handgun with him.

Ms. Ago called Ms. Diaz before the attempted holdup and arranged to have Mr. Smith meet with her to buy $200 worth of high-grade marijuana called “purple haze,” according to trial testimony.

Investigators found no weapon on Ms. Diaz's person or in her 1996 Lexus. Her wallet, which contained $129, was found in the car after she was shot. Four bags of marijuana were in an aerosol can with a false bottom that was discovered by investigators underneath the vehicle.

The jury of seven women and five men found Mr. Smith guilty under two of the prosecution's three theories of first-degree murder, that the killing was committed with extreme atrocity or cruelty and that it occurred during the commission or attempted commission of a felony, armed robbery, which carries up to a life sentence.

The jurors did not find that the slaying was carried out with deliberate premeditation.